In a dutch oven (I used a large stock pot on the stovetop), heat over medium heat & add butter and oil.
When butter/oil is heated, add onions, garlic and prosciutto.
When prosciutto has crisped up, sprinkle in flour.
Stir and cook until the raw flour smell is gone - about 2 mins.
Add mushrooms and cook until softened.
Add chicken broth and potatoes.
Allow potatoes to cook until tender.
Add green onion and slowly stir in cheese until melted.
Add cream/half & half and stir, allowing it to warm through.
Add chicken if using or serve as it with hot toasty bread.